The present invention relates to a distributed feedback semiconductor laser.
The distributed feedback semiconductor laser (hereinafter simply referred to as the "DFB laser") has, in an active layer or an adjoining layer, corrugations causing periodic refractive index variations and stably operates at a single wavelength dependent on the period of the corrugations, and hence it has excellent characteristics as a light source for high quality optical fiber communications. Furthermore, since its oscillation wavelength can be arbitrarily controlled by changing the period of the corrugations during fabrication, and since no cleavage plane is needed for a resonator unlike in conventional semiconductor lasers, it is also applicable to an integrated-optics device.
Since the prior art DFB laser has no mechanism for suppressing the TM mode in the resonator, however, it inevitably oscillates in both the TE and TM modes. Ordinarily the TM mode slightly differs in oscillation wavelength from the TE mode and its plane of polarization also differs from that of the latter, so that, in the case of transmission through a single mode fiber, it lowers the transmission capacity due to various dispersions.
On the other hand, an ordinary semiconductor laser utilizing a cleavage plane oscillates only in the TE mode because the reflectivity on the end face slightly differs in the TE and the TM mode, that is, the reflectivity in the TE mode is larger than in the TM mode. As is well-known, however, it is not subject to the axial mode control, and hence oscillates at many wavelengths.
Moreoever, in a distributed Bragg-reflector laser (DBR), a metal film is formed on a distributed Bragg reflector to cause an additional loss to the TM mode, thereby permitting oscillation in the TE mode alone.
In the semiconductor lasers, the unification of the mode of oscillation including the control of polarization plane, is also an important condition for the implementation of high quality optical fiber communications.